How to Install Plumbing Pipes for Drains

Traps hold just enough water to keep sewer gasses from entering the house.

Your home’s plumbing consists of two main parts: the water supply system and the drainage system. The water supply system in most of today’s homes is straightforward, with small pipes or tubing supplying hot and cold water, under pressure, to appliances and fixtures. The drainage system, however, is a bit more complicated and depends on both gravity and ventilation to work efficiently.

Vent-and-Soil Stack

A house’s vent-and-soil stack is a vertical pipe that connects at the bottom to the main horizontal drain line, and extends through the roof at the top end. Although the stack can be located almost anywhere, it’s generally in a central location where drainpipes can connect with ease. Ventilation pipes also connect to the vent-and-soil stack, often in the attic area.

Sloping Horizontal Drainpipes

In a typical home, a bathroom might sit quite a distance from the vent-and-soil stack. In order for the drainpipes to reach the stack, they usually run between the floor joists. It’s important to slope the drainpipes downward toward the stack at a rate of 1/4 inch per linear foot. For example, if the bathroom drain is 12 feet away from the stack, the drainpipe will be three inches lower where it connects with the stack than where it originated in the bathroom. When it comes to sloping drains, you can get too much of a good thing. If the slope is too steep, liquid can outrun solid waste, leaving it sitting in the pipe.

Drain Traps

Plumbing traps are part of the drainage system. They connect between the drainpipes and the fixtures. Traps hold small amounts of water that prevent smelly sewer gasses from flowing into the house. Sinks, tubs and showers all require a trap on the drain line. Commodes have traps built in so they require no additional trap plumbing.

Drainpipe Sizing

Building codes specify minimum drainpipe diameter, which is typically 4 inches for the main drain and the vent-and-soil stack, 3 to 4 inches for commode drains, 2 inches for sink, tub and shower drains, and 1.5 to 2 inches for vent pipes. The thing to remember when connecting drainpipes is that smaller pipes empty into larger pipes and not vice versa. For example, a 2-inch sink drain flows into a 4-inch commode drain, but a 4-inch pipe never flows into a smaller dimension pipe.

Fittings, Elbows, Reducers and Sweeps

Choosing the right fittings is imperative for connecting the drainpipes. Elbows connect horizontal drainpipes to vertical drainpipes that drop through walls before they connect to the stack. Waste flows with fewer clogs through gently sloped elbows, called sweeps, than it does through 90-degree elbows. Reducers connect small drainpipes to larger ones, and “T’s” and “Y’s” connect three lines. An example of where to use a “T” fitting would be in the wall behind a sink, where the horizontal sink drain meets the vertical drain line. The third “T” opening would accommodate a vent pipe that extends upward.

Putting it All Together

Many of today’s drainpipes are PVC or ABS pipe, but some homes have copper, steel or cast iron pipes. The most user-friendly are PVC and ABS, requiring only a pipe saw to cut and special cleaner, primer and glue to connect the fittings. Before altering or installing new drains, contact your local building authority for a permit, and find out if a licensed plumber must do some or all of the work.

About the Author

I've written hundreds of how-to articles on a wide range of subjects. I can produce quality text consistently. I am familiar with AP Style writing and the need to slant it to fit the web with keyword-centric content. In addition to how-to articles, I am experienced in journalistic pieces and copywriting. I would appreciate a chance to learn more about your company and demonstrate my writing skills. Thank your for your time and consideration. Sincerely, Glenda Taylor